(a) Field of the Invention
The present invention relates to a semiconductor for generating an electromagnetic wave, and more particularly, to a far-infrared electromagnetic wave generating semiconductor using phonons.
(b) Description of the Prior Art
It is known that each atom in a semiconductor tends to oscillate about an equilibrium position due to the interplay of thermal energy and the forces which bind together the atoms in the semiconductor crystal. The thermal energy of the semiconductor is considered to be contained in certain combinations of partical vibrations called normal modes. Each normal mode contains a discrete quanta of energy, E=h of .omega. where .omega. is the frequency of the mode and h is planks constant divided by 2.pi.. These quanta of energy are referred to as "phonons".
There are two types of phonons, acoustical phonons and optical phonons. Acoustical phonons represent motion of atoms, similar to vibrations obtained when a sound wave is propagated through the crystal. Optical phonons represent motion in which the center of mass of the different atoms in a molecule remains fixed, but the atoms in the molecule move relative to each other. Phonons also exhibit two different modes, longitudinal (LO) where the atoms vibrate in the same direction as phonon movement, and transverse (TO) where the atoms vibrate perpendicular to the phonon motion.
The present inventors have already proposed an electromagnetic wave generator which can generate coherent electromagnetic waves having a wavelength in the range of from the submillimeter region to the far-infrared region (a wavelength from 10.mu. to 1 mm) using lattice vibration, i.e. phonons, in a semiconductor.
Coherent electromagnetic waves have been generated by exposing a semiconductor to a beam of incident light having a frequency .omega..sub.o to cause excitation of lattice vibration .omega..sub.r in the semiconductor and Raman scattering .omega..sub.s,a =.omega..sub.o .-+..omega..sub.r. This Raman scattering, in turn, causes lights indicated by .omega.s,a. Thus, a coherent lattice vibration .omega..sub.r is excited due to the oscillation of the photons to produce a simultaneously coherent electromagnetic wave.
Drift velocity of electrons (or holes) v.sub.d can be increased by applying either a high direct current voltage or a microwave voltage across a semiconductor until it reaches the condition that optical phonons (quantum energy, h.omega..sub.r), i.e., 1/2mv.sub.d.sup.2 -.perspectiveto.h, .omega..sub.r are excited. Since .omega..sub.r is substantially constant, the drift velocity of electrons is almost constant, i.e., velocity is saturated. Under such a conditions, many phonons are vigorously produced. However, the wavelength of the phonons produced by the collision between free electrons and lattices is shorter than that of a light wave so that it is not easy to achieve a phase-matching therebetween. Moreover, most of the phonons are longitudinal optical (LO) phonons, where which cannot interact with far-infrared electromagnetic wave. On the other hand, in the case of collisions of free electrons with impurities, the problem of phase-matching does not occur and, therefore, the drawback that the wavelength of the produced phonons being shorter than that of a far-infrared electromagnetic wave can be avoided. In addition, the phonons which are produced are not limited to LO type phonons.
On the othr hand, U.S. Pat. No. 3,611,180 suggests a far-infrared laser in which free electrons and positive holes in a non-polar semiconductor, such as Ge or Si, are accelerated to produce a collision excitation of an impurity. That is, the electrons are excited to the energy levels of the excited states of those in the impurity and, when the electrons of the impurity descend to lower levels, photons of far-infrared light are emitted. However, a quantum transition between the shallow energy levels in a semiconductor generally has an extremely low efficiency of emitting photons, and alternatively will emit phonons, and therefore has a high probability of a radiationless transition. The reason is that the electrons caught in the shallow energy level move in a much larger radius than the lattice distance in the semiconductor due to a weak connection with the impurities. Thus, the collision with crystal lattices freely occurs and a lattice vibration will be generated, that is, the phonons will be excited before the photons are emitted. Accordingly, the far-infrared laser using impact excitation of the impurity has been merely proposed without being realized.
Also, in the generation of far-infrared light, the absorption coefficients of the free electrons and positive holes become large in proportion to .lambda..sup.2 in respect of a wavelength .lambda.. That is, in the far-infrared light having a wavelength of 20 .mu.m, the free carrier absorption is large, e.g., as much as 400 times greater than with near-infrared light having a wavelength of 1 .mu.m. Thus, it has been difficult to substantially amplify a far-infrared electromagnetic wave. Further, the energy of the shallow level in Ge and Si is very small and is substantially ionized at room temperature. It, therefore, has never been used to generate far-infrared light.
It is an object of the present invention to provide a far-infrared electromagnetic wave generator without the above drawbacks. That is, a polariton mode wave particular to a polar semiconductor, unlike Ge and Si, is used and, although the energy produced by the transition between impurities is mainly a phonon, the polariton mode is amplified by positively utilizing the phonon to generate the far-infrared electromagnetic wave.